Final answer:
Valence electrons determine the chemical properties of an element. Elements in the same group share similar chemical properties due to their identical valence shell electron configuration. Elements in a row exhibit gradual changes in chemical properties as their valence shell electron configuration gradually changes.
Step-by-step explanation:
The valence electrons mainly determine the chemical properties of the elements. The elements in the same group have similar chemical properties because they have the same valence shell electron configuration. The elements in a row show a gradual change in chemical properties because their valence shell electron configuration changes gradually along the row.